1. Field of the Invention
Exemplary embodiments of the present invention relate to an envelope detector, and more particularly, to an envelope detector detecting an envelope of a digital modulation signal.
2. Description of Related Art
A wireless communication system using an RF signal is configured to include a transmitter and a receiver. The transmitter modulates a digital signal to be transmitted into the RF signal and transmits the modulated RF signal to the receiver and the receiver receives the RF signal and recovers the received RF signal into an original digital signal.
Hereinafter, referring to FIG. 1, a signal processing process in a generally used heterodyne receiver will be described below. The RF signal received through a receiving antenna 100 is input to an RF terminal of a mixer 130 through a band pass filter (BPF) 110 and a low noise amplifier 120. The RF signal is mixed with a signal from a local oscillator 135 in the mixer 130 and is frequency-down-converted into an intermediate frequency (IF) band that is a difference between a frequency of the RF signal and a frequency of an LO signal. The IF signal passes through the band pass filter 140 and the amplifier 150 and is then input to an envelope detector 160. The envelope detector 160 detects a power level of an input signal input to the envelope detector 160 and an envelope of a modulated signal. A low pass filter 170 removes harmonic component of the signal output from the envelope detector 160. Meanwhile, the signal passing through the low pass filter 170 is recovered to an original digital signal through a decision circuit and amplifier 180.
In the signal processing process as described above, the envelope detector 160 may detect the envelope of the modulated signal in various types according to a modulation type of a transmitted modulation signal.
Hereinafter, an envelope detection method will be described with reference to FIGS. 2 to 4. As illustrated in FIG. 2, a detector 200 is a simplest and frequently used detector that is configured to include a diode 210, a capacitor 230, and a resistor 250. The detector 200 illustrated in FIG. 2 has a disadvantage in that detection sensitivity is relatively low.
A detector 300 illustrated in FIG. 3, which is an ASK envelope detector, detects an envelope using a mixer 310. The mixer 310 used for envelope detection includes both of a passive mixer using a diode that is a passive device and an active mixer using a field effect transistor (FET) or a bipolar junction transistor (BJT) that is an active device. When the envelope is detected using the mixer 310, a local oscillator 350 generates a sine wave having the same frequency and phase as a carrier and inputs the generated sine wave to the mixer 310. The sine wave generated from the local oscillator 350 is multiplied by a modulated signal input to the mixer 310, such that a baseband signal and a frequency component having a frequency twice higher than a carrier frequency are output from the mixer 310. The type is a coherent detection type, wherein the coherent detection type has advantages in that detection sensitivity characteristics are excellent and a dynamic range is wide. Meanwhile, for the coherent detection, the frequency and phase input to an input terminal 330 of the mixer is the same as the frequency and phase of the signal from the local oscillator 350 and therefore, the coherent problem is not easy.
A detector 400 illustrated in FIG. 4 uses a type similar to FIG. 3, but in order to solve the coherent problem between the input signal and the signal from the local oscillator, the local oscillator is not separately used and the input signal is branched and used. The input signal is input to an input terminal 430 and at the same time, is input to the mixer 410 using an amplifier chain 450, and as a result, the same effect as using the local oscillator 350 of FIG. 3 can be obtained.
Both of the detectors 300 and 400 illustrated in FIGS. 3 and 4 are similar to each other in that signal power different from the input signal is supplied to one terminal of the mixer and include the local oscillator 350 or the amplifier chain 450, thereby causing a problem of increasing a volume of the receiver and increasing manufacturing costs of the receiver.